Method of operating radio receiver implemented in a single CMOS integrated circuit

ABSTRACT

A single chip superhetrodyne AM receiver is disclosed herein. To compensate for process variations in the implementation of the IC, bias currents setting the operating conditions for various amplifiers and other components in the system are adjusted based on frequency control signals in a PLL circuit in the local oscillator. Since the magnitude of the control signal reflects the process variations, the bias currents are adjusted based on the control signal to offset these variations in other portions of the receiver. To further improve the signal to noise ratio of the receiver, the IF filter is tuned within a range so as not to include any integer multiple or integer divisor of the timing reference frequency. Various techniques are described for enabling a complete superhetrodyne AM receiver to be implemented on a single chip which receives an antenna input signal and outputs a digital data signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/661,879,filed Sep. 11, 2003, entitled “Method Of Operating Radio ReceiverImplemented In A Single CMOS Integrated Circuit”, which is a divisionalof application Ser. No. 09/734,844, filed Dec. 11, 2000, entitled “RadioReceiver In CMOS Integrated Circuit”, which is a divisional ofapplication Ser. No. 09/495,323, filed Jan. 31, 2000, now U.S. Pat. No.6,278,866, issued Aug. 21, 2001, entitled “Bias Signal Generator InRadio Receiver”, which is a divisional of application Ser. No.09/075,281, filed May 8, 1998, now U.S. Pat. No. 6,167,246, issued Dec.26, 2000, entitled “Fully Integrated All-CMOS AM Receiver”, which isbased on the provisional application Ser. No. 60/046,023, by J. ScottElder, et al., filed May 9, 1997, entitled “Fully Integrated All-CMOS AMReceiver”, all of which applications are incorporated herein byreference in their respective entireties. Priority is hereby claimed toall of the above-listed applications.

FIELD OF THE INVENTION

This invention relates to radio wave receivers and, in particular, to areceiver formed as a single integrated circuit.

BACKGROUND

Radio receivers, such as amplitude modulation (AM) type receivers, arewell known. It is desirable for cost and size reasons to implement suchreceivers on a single integrated circuit chip. However, obstacles haveprevented a practical AM receiver from being implemented on a singlechip.

SUMMARY

A single chip superheterodyne AM receiver is disclosed herein. Pin countmay be 4 or more. The receiver is a OOK (ON-OFF keyed) Receiver IC forremote wireless applications. This device is an “antenna-in, data-out”monolithic device. All RF and IF tuning is accomplished automaticallywithin the IC, which eliminates manual tuning, and reduces productioncosts. Receiver functions are completely integrated. The result is ahighly reliable yet extremely low cost solution for high volume wirelessapplications. Because the receiver is a true single-chip radio receiver,it is extremely easy to apply, minimizing design and production costs,and improving time to market.

The receiver uses a novel architecture that allows the receiver todemodulate signals over a wide RF band, which eliminates the need formanual tuning. This is referred to as a swept LO mode. This alsosignificantly relaxes the frequency accuracy and stability requirementsof the Transmitter, allowing the receiver to be compatible with bothSAW-based and LC-based transmitters. The receiver sensitivity andselectivity are sufficient to provide low bit error rates for decoderanges over 100 meters, equaling the performance of other more expensivesolutions.

All tuning and alignment are accomplished on-chip with a referencefrequency provided by a low-cost ceramic resonator or an externallysupplied clock reference. The receiver performance is insensitive todata modulation duty cycle. The receiver may be used with such codingschemes as Manchester or 33/66% PWM.

To prevent noise from the clock reference decreasing the sensitivity ofthe receiver, the IF filter is tuned such the no integer multiple orinteger divisor frequency of the timing reference occurs in the IF passband.

All post-detection (demodulator) data filtering is provided on thereceiver chip, so no external filters need to be designed. Any one offour filter bandwidths may be selected externally by the user.Bandwidths range from 0.6 kHz to 4.8 kHz in binary steps.

The various filters and demodulator have frequency characteristics basedon the output of an internal timing generator which receives clocksignals from an external reference. Therefore, tuning of the filters anddemodulator may be accomplished by changing the external referencefrequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a single chip receiver in accordance withone embodiment of the invention.

FIG. 2 provides additional detail of the LO Sweep Generator.

FIG. 3 illustrates an optimum LO sweep range.

FIGS. 4-76 illustrate actual circuitry for implementing a preferredembodiment of the single chip receiver. FIG. 4 identifies the functionalelements from FIG. 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 illustrates the basic structure and elements of one embodiment ofthe invention, a fully integrated all-CMOS AM receiver 20. Pins or padsof the integrated circuit are shown outside of the dashed outline. Thereceiver is a superheterodyne type, which means the incoming modulatedRF signals are preamplified and then mixed with a local oscillatorfrequency. The mixed signal is then filtered to generate a modulatedintermediate frequency (IF). The IF signal is then amplified anddemodulated, and the data extracted.

All elements are constructed using Complementary Metal OxideSemiconductor (CMOS) technology. CMOS technology also permits certainbipolar elements like diodes, transistors, and others to be combined inthe receiver. Where appropriate, these devices are utilized. Theelements of the system are as follows, with associated reference number:

Functional Element Reference No. a. Radio Frequency (RF) or HighFrequency (HF) 1 Preamplifier b. RF Mixer or Translator 2 c.Intermediate Frequency (IF) Amplifier 3a, 3b d. Intermediate Frequency(IF) Filter 4 e. Peak or Envelope Detector 5 f. Baseband Filter 6 g.Data Slicer or Comparator 7 h. DC Extractor 8 i. Local Oscillator (LO) 9j. LO Sweep Generator  9a k. Reference Oscillator 10  l. 2 Vt BiasSupply 11  m. Automatic Gain Control (AGC) 12  n. Filter Tuning 13  o.Timing Generator 14  p. Bandgap Reference 15 

In the context of this application, fully integrated means that all ofthese functions in their entirety have been simultaneously incorporatedonto a single semiconductor die (integrated circuit or IC). Additionalaspects of the receiver (to be detailed subsequently) reduce overallradio system complexity, cost, and transmitter performance requirements.Note that DC control lines for the receiver (described subsequently) maybe either pinned-out for maximum end-user control, or may be fixed onthe die via a metal mask, the latter allowing the most economicalpackaging.

The entire RF and IF signal path of the receiver employs a balanced,differential design approach, which is required for stability, powersupply rejection, and decoupling of the RF circuits from the IF andbaseband signals. Additionally, all AC currents are balanced to mitigateany AC terms on the bias supply.

Brief Description of the Component Features

The Internal Reference Oscillator 10, with an externally applied timingelement or timing signal, supports a multiplicity of carrierfrequencies. The timing element may be any known reference frequencygenerator, such as a crystal oscillator. The receiver is carrierfrequency independent.

The Peak Detector 5 and Baseband Filter 6 time constants of theirrespective CR circuits are related to the Reference Oscillator 10frequency and are selectable by one or more DC (logic) control linesSEL0, SEL1. The receiver can thus support a multiplicity of data rates,and the receiver is data rate independent. The time constants for thesefunctions are highly precise and are independent of temperature andsemiconductor processes.

The IF Filter 4 and Baseband Filter 6 are fully integrated on the IC.The result is an implementation which may be physically placed in an8-pin IC package.

A package configuration as low as 4 pins is possible by modifying thereceiver in the following ways. Firstly, use only one IC pin for powersupply, and one IC pin for ground. Change the DC Extractor 8 design to aswitched-capacitor lowpass filter, allowing integration of capacitor C1.Secondly, the AGC 12 function may be designed using a digital-to-analog(DAC) based structure, which allows integration of capacitor C2.Similarly, any pin configuration from 4 to 8 is possible, depending onthe level of integration that the constructor chooses to use.

The Local Oscillator 9 (LO) can be operated in either fixed mode orswept mode, selectable via a DC control line SWEN. Fixed mode operationis preferred for precision or high-performance applications.

In Swept mode, the LO frequency is varied across a range of frequenciesat a rate sufficiently higher than the data rate to allow for peak(envelope) detection. This mitigates the requirement for an accuratelycontrolled and/or age and temperature stabilized transmitter carrierfrequency.

The LO Sweep Generator 9 a is a (÷m/÷m+1) phase-locked-loop (PLL) wherethe loop is forced to operate half the time at ÷m and the other half ofthe time at ÷m+1. The LO Sweep Generator 9 a uses a (stable) biascurrent based on the Reference Oscillator 10. The sweep signal is thus acontrolled excursion waveform with either a ramp (current into acapacitor) or resistor-capacitor (RC) response characteristic. Thisguarantees a minimum time of the mixer IF output signal within the(integrated) IF filter frequency response.

The LO Sweep Generator PLL employs a temperature compensated bandwidthscheme to linearize the LO sweeping process. LO sweep characteristic isthus independent of temperature and semiconductor processing.

The IF amplifier 3 a, 3 b design provides a bandpass characteristic,with no DC gain. This allows direct amplifier coupling without DC offsetcorrection or coupling capacitors. Further, the bandpass characteristicsare stabilized to the Reference Oscillator 10 for temperature andprocess independence. Other, less desirable embodiments include (1)capacitive coupling between amplifier stages, and (2) DC coupling withoffset cancellation.

The IF signal is AC-coupled from IF amplifier 3 b to Peak Detector 5,eliminating any DC-offset related peak detector time constant errors.

The Peak Detector 5 is configured as a closed-loop voltage followerdriving an open-loop peak detector. Both circuits use matched averageload currents to remove DC errors. This configuration provides fasterattack time on peak detection. Alternative embodiments are either asingle closed-loop or open-loop peak detector, or conventional envelopedetector.

The RF Preamplifier 1 and RF Mixer 2 are biased with LO Oscillator 9based currents to achieve temperature and process independence. In oneembodiment, the bias is based on the magnitude of a VCO control signalused to generate the LO frequency. Since process variations change thecharacteristics of components (e.g., transistors) across the entirechip, the signal controlling the VCO reflects variations in otherfunctional units. Hence, the control signal may be used to adjust themagnitudes of bias currents for other functional units to offset theeffects of process variations.

Back-to-back non-rectangular drain FET structures are used at criticalhigh frequency nodes to minimize capacitance and improve bandwidth.Various embodiments of the structure are circular, hexagonal, octagonal,etc. Circular is preferred.

The RF preamplifier 1 output is taken from the second stage of a (threestage) feedback amplifier, where gain-bandwidth product is maximum. Analternative embodiment is to take the signal from the first or thirdstages.

A 2Vt bias supply 11 is constructed on the IC for all logic circuits.This mitigates logic switching noise which allows further logic functionintegration without impacting receiver sensitivity due to IC or powersupply noise. Alternative embodiments are to build supply voltagesranging from 2Vt to the positive supply voltage. (Here Vt refers to thethreshold voltage of a FET transistor.)

The Data Comparator 7 output stage is current limited, which reducesoutput switching noise.

The DC Extractor 8 provides a transfer impedance related to thereference oscillator, providing a very precise temperature and processindependent value. The transfer impedance also varies with data rate, asselected by DC control lines SEL0, SEL1, so that the Data Comparator 7slicing threshold voltage time constant adjusts automatically with datarate.

The RF Preamplifier 1 input is ac-coupled to tailor the lower cutofffrequency to provide additional rejection of lower frequencyinterference (e.g., from FM transmitters).

The Bandgap Reference 15 provides a temperature and power supplystabilized set of reference voltages which are used to set certaincritical bias points within the invention.

Operation of Receiver of FIG. 1

System Operation—Introduction

The Fully Integrated All-CMOS AM Receiver 20 converts amplitudemodulated (AM) signals at its input (Antenna In of RF Preamplifier 1)into the equivalent (data) code used at the transmitter to construct theAM signal.

The radio frequency (RF) and intermediate frequency (IF) portion of thereceiver functions similarly to a superheterodyne receiver. Specificallythis includes RF

Preamplifier 1, RF mixer 2, IF Amplifiers 3 a,3 b, and IF filter 4.

The baseband portion of the receiver functions similarly to an AM or OOK(On-Off Key) demodulator. Specifically, this includes the Peak Detector5, Baseband Filter 6, DC Extractor 8, and Data Slicer or Comparator 7.

The Local Oscillator (LO) 9 and LO Sweep Generator 9 a provide thenecessary LO for frequency translation. The LO Sweep Generator 9 a hasseveral operating modes as will be detailed subsequently.

The Reference Oscillator 10 provides a stable reference frequency forthe receiver when the Reference Oscillator's input is attached either toa precise timing element or a precise timing signal (i.e., externalclock input). Examples of timing elements are crystals, ceramicresonators, or a resonant inductor-capacitor (LC) circuit.

The 2Vt Bias Supply 11 provides the minimum bias necessary to operateconventional CMOS logic circuits without “shoot-through” logic switchingcurrents. This allows the fastest operation logic without power supplyswitching noise.

An Automatic Gain Control function 12 is also provided to extend therange of input power levels the receiver can handle before overloadoccurs.

All of the functions illustrated in FIG. 1 are simultaneously integratedonto a single Complementary Metal Oxide Semiconductor (CMOS) die to forma single integrated circuit (IC).

RF Signal Reception

RF Preamplifier 1

The elements of the receiver which process the incoming modulated signalare the RF Preamplifier 1, the RF Mixer 2, the Local Oscillator (LO) 9,and the LO Sweep Generator 9 a.

The RF Preamplifier 1 provides signal gain at the carrier frequency. Thesignal applied to the RF Preamplifier input from the signal source(usually an antenna) is AC coupled with low frequency roll-off tailoredto lie between (lower) Frequency Modulated (FM) sources and the lowerend of the Federal Communications Commission (FCC) periodic band. Thisis done to minimize interference from lower frequency sources andmitigate the need for additional input filtering.

The RF Preamplifier 1 is biased using a current related to a currentgenerated in the Local Oscillator 9 a. In other words, the magnitude ofthe bias current or voltage used to set the operating conditions of thePreamplifier 1 is related to the current or voltage supplied to acontrol input of a voltage controlled oscillator (VCO) for generatingthe LO frequency. Thus, process variations that similarly affect theLocal Oscillator 9 a frequency and the RF Preamplifier 1 are compensatedby the current or voltage into the VCO and the bias current or voltageused to set the operating conditions of the RF Preamplifier 1. Thisguarantees that the RF Preamplifier gain and bandwidth are temperatureand process independent.

Back-to-back non-rectangular drain Field Effect Transistor (FET)structures are employed at critical high frequency nodes of the RFPreamplifier to minimize capacitance and improve bandwidth. Thisstructure has important ramifications, since the result is a very smallcapacitive parasitic at the drain node of the FET and a largercapacitance at the source node of the FET. This breaking apart of thenormally equal drain and source parasitic capacitances actually furtherimproves the gain-bandwidth (GBW) of the amplifier beyond just theimprovement anticipated by a lowering of the drain capacitance. Saidanother way, the extra source capacitance actually improves GBW.

In addition, the output of the RF Preamplifier 1 is taken from thesecond stage (FET gate) of the output follower that comprises themulti-stage RF Preamplifier 1. This is the node in the RF Preamplifierwith the greatest bandwidth.

High gain-bandwidth performance in CMOS dictates the use of smallchannel-length FET's, which suffer from low early voltage. A closed loopbias control circuit is also provided in the RF preamplifier whichcompensates for low early voltage and improves yield of the invention.

Integration of the RF Preamplifier onto the integrated circuit in CMOSproduces a very low capacitance circuit which maximizes any LO returnloss back to the antenna, since LO feedthrough back to the (antenna)input is undesirable.

RF Mixer 2

RF Mixer 2 provides frequency translation of the RF carrier which hasbeen amplified by RF Preamplifier 1. For an input frequency (Frf) andLocal Oscillator (LO) frequency (Flo), the mixer 2 generatestheoretically two new frequencies, Frf+Flo and Frf−Flo. The second termis subsequently processed by the IF signal processing circuits IFAmplifier 3 a, 3 b, and IF Filter 4. The mixer's high frequency responseis tailored in order to effectively filter out the first mixer term.

A CMOS mixer is used since (1) such a structure provides a more linearmixing characteristic, and (2) the physical construct is very compact,minimizing capacitances and maximizing LO feedthrough loss back to theantenna. To further improve the linearity of the system and extend itsability to operate at high input signal levels, the mixer is designed toinclude automatic gain control (AGC). Thus for input levels higher thanthe present state of the art, the receiver remains linear and continuesto function as an

AM Receiver and Demodulator.

The RF Mixer 2 is biased using a current which is tuned to the LocalOscillator 9 a. This guarantees that the RF Mixer gain and bandwidth aretemperature and process independent. It is known to provide a verticaltree structure, where the RF preamplifier, RF Mixer, and LO Oscillatorare vertically aligned, so as to share a common bias current and improvepower efficiency. The approach in the present regulator of FIG. 1 is tobias the RF Preamplifier 1 and RF Mixer 2 from currents mirrored off ofthe Local Oscillator, which is PLL based. The value of this approach isto track out temperature and semiconductor processing variablesassociated with the use of CMOS technology.

Special back-to-back non-rectangular drain Field Effect Transistor (FET)structures are employed at critical high frequency nodes of the RF Mixer2 to minimize capacitance and improve bandwidth. This structure hasimportant ramifications, since the result is a very small capacitiveparasitic at the drain node of the FET and a larger capacitance at thesource node of the FET. This breaking apart of the normally equal drainand source parasitic capacitances actually further improves thegain-bandwidth (GBW) of the mixer beyond just the improvementanticipated by a lowering of the drain capacitance. Said another way,the extra source capacitance actually improves GBW.

Local Oscillator (LO) 9 and LO Sweep Generator 9 a

A CMOS RF Oscillator circuit 21 (FIG. 2) provides the LO frequency forthe frequency translation process accomplished within RF Mixer 2. The RFOscillator 21 is a component of the LO Sweep Generator 9 a.

Reference will be made to FIG. 2, illustrating the LO Sweep GeneratorSubsystem, which provides an expanded illustration of the LocalOscillator 9 and LO Sweep Generator 9 a from FIG. 1.

The LO Sweep Generator 9 a is in actuality a phase-lock-loop (PLL). ThePLL generates a signal which is an integer multiple of the ReferenceOscillator 10 frequency. The PLL operates as follows. The RF Oscillator21 output frequency is divided by M or M+1 (depending on a control inputSWEN) by Divider 22. This new lower frequency is compared against theReference Oscillator 10 frequency by Phase Detector 23. Phase Detector23 outputs on line 27 both a dc voltage and an ac voltage. Only the dcterm is of interest as its value signifies the difference in frequencybetween the two input frequencies of Phase Detector 23. The ac voltageis filtered via Loop Filter 24, extracting the dc value identified asFctrl 28. The natural tendency of the PLL is to force the dc value ofFctrl to whatever value is required such that the two frequencies intothe Phase Detector 23 are identical.

Signal SWEN 26 dynamically alters the division factor within Divider 22.SWEN is typically a two state signal (Logic 0 or Logic 1). If SWEN isheld fixed at one logic state, the division factor is M, and the LOfrequency is the Reference Oscillator 10 frequency multiplied by M. IfSWEN is held fixed at the other logic state, the division factor is M+1,and the LO frequency is the Reference Oscillator 10 frequency multipliedby M+1. If SWEN transitions dynamically between its two logic states,the LO will transition smoothly and continuously from fl to fu, where flis the lower frequency, and fu is the upper frequency of the LOexcursion.

Within the receiver, the Local Oscillator 9 may be operated in either oftwo modes (1) fixed LO, or (2) swept LO. In fixed mode, the LO frequencyis held fixed and the receiver functions as a conventionalsuperheterodyne AM receiver. In such a case, the transmitted RF carrierfrequency must fall within the passband of the IF Filter, dictating someconstraints on transmitter frequency alignment and age/temperaturestability.

Alternatively, in Swept LO mode, the receiver sweeps the LO fromfrequencies fl to fu, as explained just previously. This effectivelysweeps the entire RF band in the vicinity of the range fl to fu into theIF filter passband. This swept IF is then processed by the receiver in afashion to recover the AM envelope just as would occur in theconventional fixed LO superheterodyne receiver. The consequences of suchoperation is that the constraints on transmitter frequency alignment andstability are significantly relaxed.

In one embodiment, the sweep mode results in downconversion of allsignals in a band 2-3% around the transmit frequency.

An alternative means of sweeping the LO to that described above is todrive the Reference Oscillator 10 input with a swept frequency precisiontiming signal. Such an approach would mitigate the need for dual-modulesdivision within the LO Sweep Generator PLL.

Reference will again be made to FIG. 1.

IF Signal Processing

IF Amplifier 3 a,3 b

In the receiver, an IF Amplifier 3 a connects the RF mixer 2 and the IFFilter 4. The IF Amplifier 3 a amplifies the (frequency translated)signal from the RF mixer 2. An IF Filter 4 selectively extracts the IFsignal of interest, which is again amplified by an identical IFAmplifier 3 b. Each IF Amplifier 3 a/3 b is a fully balanced,differential design, required to provide power supply rejection andminimize coupling between the RF and IF sections of the receiver acrossthe common power supply connection.

The IF Amplifier 3 a/3 b design provides a bandpass frequencycharacteristic with no DC gain. This allows the IF Amplifier to bedirectly coupled to other elements of the receiver, like directly to theIF Filter 4 without the need for DC offset correction or couplingcapacitors. Further the IF Amplifier bias currents are derived from theReference Oscillator 10 frequency. The result is that the bandpassfrequency response of the IF Amplifier 3 a/3 b is constant withtemperature and semiconductor process variations. The IF amplifier gainis set by connecting the amplifier's signal p-channel FET transistors top-channel FET transistors as active loads. The resultant gain is theratio of the gm of the signal FET divided by the gm of the load FET,which is independent of temperature and bias current. The result isconstant amplifier gain independent of temperature and semiconductorprocess variations.

The IF Amplifier 3 a/3 b also provides gain variation as a function of asignal applied externally to the IF Amplifier. This is commonly referredto as Automatic Gain Control or AGC. AGC is necessary in the IFAmplifiers to allow the system to remain linear and operable when verylarge input signal levels are applied to the receiver's antenna input.The AGC CTRL function 12 detects the condition of excessive input signaland generates a counter compensating voltage which reduces the gain ofother stages in the RF and IF paths, namely the RF Mixer 2, IFAmplifiers 3 a/3 b, and IF Filter 4. The IF Amplifier 3 a/3 b isconstructed in CMOS, rather than bipolar technology, which alsoincreases the range of linear operation of amplification.

Finally, the IF Amplifier 3 a/3 b design employs pFET transistors ratherthan nFET transistors in the signal path of the IF Amplifier, as pFETdevices provide more linear operation than nFET devices.

IF Filter 4

An IF Filter 4 is integrated onto the receiver chip. The function of theIF Filter 4 is theoretically to reject all signals except the frequencyFrf-Flo output from the RF Mixer 2. The frequency response of thepreferred embodiment is a bandpass characteristic, with the highestquality factor (Q) that can be constructed in integrated form. Becausequality factor is inversely related to filter bandwidth, as the qualityfactor rises, the bandwidth shrinks, where the ideal case would be aquality factor of infinity. This would result in a system that was (intheory) perfectly selective.

Problems one encounters with integrated filters is the variation infrequency response characteristics with temperature and semiconductorprocess variations. In the receiver, we use a gm-c filter structure,which is preferred since the frequency response characteristics of thisstructure can be readily tuned by using a PLL whose reference input isour stable Reference Oscillator 10 frequency. The function labeledFilter Tuning 13 is the PLL which builds the bias current thatstabilizes the IF Filter 4 frequency response characteristic. Theadjustment of bias currents based on currents generated in a PLL hasbeen discussed previously. Thus the IF Filter 4 bandpass responsecharacteristic is stabilized against temperature and semiconductorprocessing variations. Additionally, the tuning current used to bias theIF Filter 4 is related to the tuning current used to bias the IFAmplifiers 3 a/3 b. This guarantees that the frequency responsecharacteristics of the IF Filter and IF Amplifiers track with processand semiconductor variations.

The IF Filter 4 bandpass response characteristic is also tuned so thatno integer multiple or integer divisor frequency of the external timingreference occurs in the IF passband. In one embodiment, the timingreference is 3 MHz and the IF filter 4 is tuned to a center frequency of2.25 MHz. This band does not include any integer multiple or integerdivisor of 3 MHz.

Location of the IF Filter 4 within the receiver is important; the IFFilter 4 is placed between the IF Amplifiers 3 a and 3 b. This providesisolation between the IF Amplifiers, important to construct largeamounts of stable IF gain.

The IF Filter 4 structure is completely balanced and differential, whichmaximizes power supply rejection and noise coupling into the powersupplies. Full integration of the IF Filter also reduces parasiticcapacitances, which reduces filter signal feedthrough. This isequivalent to stating that the ultimate out-of-band rejection of the IFFilter is greater than that found in discrete implementations.

IF Filter Quality Factor Vs. Swept LO

Although high Q in the IF Filter 4 is desirable to improve receiverselectivity, higher values of Q will force the transmitter frequency tobe correspondingly more accurate and stable with age and temperature.For our receiver, where the receiver provides a swept LO mode as well asa fixed LO mode, there is a further consideration. As discussed earlierin regards to the LO Sweep Generator 9 a, in the swept LO mode, the LOfrequency is varied smoothly and continuously between two frequencies fi(lower frequency) and fu (upper frequency). This effectively translatesa band of frequencies around the RF carrier frequency into the IFfrequency response of the system.

If an AM carrier appears on the system (antenna) input withinfrequencies fi and fu, it will be translated into the IF spectrum. Byusing a sufficiently fast sweeping LO and a peak detector for theenvelope detection process with properly adjusted attack and decay timeconstants, this signal will be recovered with great efficiency. As thesweep rate increases toward infinity, the system response approachesthat of a continuous or fixed LO, in which case the full modulationenvelope is recovered. Conversely, sweeping the LO too slowly results inloss of signal due to droop associated with the peak detector decay timeconstant and results in a corresponding reduction in signal-to-noiseratio. However, for a given IF Filter Q, if the LO sweep is too fast,the response time of the IF filter becomes the determining factor inrecovered modulation envelope. Or, said another way, there is someoptimum sweep speed that provides the best system performance for agiven Q.

The sweep speed can be normalized into the (average) number of timesthat the LO sweeps from fi to fu over a data bit time of the data thatis being recovered. Analysis identifies that this optimum range isbetween 4 and 10 sweeps (or hits) per bit time for the particular Q usedin the IF Filter 4. The system performance outside this range issignificantly inferior to the performance within this range. Thereceiver uses 7 hits per data bit as the preferred value.

The optimum and preferred LO sweep waveform is a ramp waveform, so thateach frequency translated into the IF spectrum has equal time within thepassband of the IF Filter. Since the LO is generated within a PLL, thereare difficulties with generating such a waveform that results inprecision in the frequency end points (fi and fu) and in the sweep rate(Hertz/volt or Hertz/mA) of the oscillator generating the instantaneousLO frequency.

An easier alternative used in the receiver, which produces acceptableresults, is to use a resistor-capacitor (RC) waveform, provided that thewaveform rise and fall times do not exceed certain minimum and maximumvalues over process and temperature variations. Such a task isaccomplished by the LO Sweep Generator 9 a, which is discussedsubsequently. Basically, the rise and fall times must be slow enough sothat at the points of maximum rate of change of the waveform, theeffective rate of change of the frequencies being swept into the IFspectrum does not exceed the IF Filter 4 response time set by Q.Likewise, the rise and fall times must be fast enough that the PLLgenerating the LO can get from fl to fu, to guarantee that the receiversweeps across (or looks across) the frequency ambiguity of thetransmitter. The LO Sweep Generator design of FIG. 2 provides a PLLbandwidth which is temperature compensated and process independent,centered nominally between these minimum and maximum values.

Optimum LO Sweep Range

Reference will be made to FIG. 3, relating to an optimum LO sweep range,which illustrates LO sweep range selection.

As pointed out earlier, there are relationships between IF Filter Q andcenter frequency, the number of “hits” or sweeps performed per bit, andthe LO sweep rate. Faster sweeps allow higher data rates, with the upperlimit being the response time of the IF Filter 4. Thus, the sweep rangecannot be chosen arbitrarily. Greater sweep ranges reduce the “hit”rate, or force faster LO sweep rates, which lowers IF Filter in-bandtime. FIG. 3 illustrates the optimum placement of fl and fu, the lowerand upper LO frequencies, respectively.

Consider firstly that the transmit frequency has its own range ofambiguity, from Frfl to Frfu. If the transmit frequency is at Frfu, thenthe LO must sweep up to within the IF frequency of Frfu, denoted Fu.Similarly, if the transmit frequency is Frfl, the LO must sweep down towithin the IF frequency of Frfl, denoted Fl. Sweeping any further onlypulls in extra spectrum unnecessarily and impacts the sweep rate. Theoptimum relationship is shown in FIG. 3. Here

Fl=Frfn−Famb/4

Fu=Frfn+Famb/4

where Frfn is the nominally set transmit frequency, and Famb is the fullamount of transmit frequency ambiguity, based on transmittermisalignment, aging, temperature, and so forth.

An alternative approach to sweeping the LO is to sweep the IF Filter 4.This could be accomplished by providing an electronic means to controland vary the IF Filter center frequency and should yield the sameperformance as sweeping the LO.

Reference will again be made to FIG. 1.

Baseband Signal Processing

Peak Detector 5

When the receiver is in the Swept LO mode of operation and an AMmodulated RF carrier appears on the system (antenna) input, the signalout of IF Amplifier 3 a is a sequence of frequency bursts during thetime period that the transmitter transmits a mark (e.g., a digital one).Each burst represents the action of the LO sweeping the RF carrier intothe IF Filter 4 passband. Similarly, when the receiver is in the fixedLO mode of operation, the IF Amplifier 3 a output is a continuous toneat frequency Frf-Flo for the time period that a transmit mark persists.The receiver employs a Peak Detector 5 to recover the AM modulation,since the information is conveyed in the peak of the output of the IFAmplifier in both cases.

Important aspects of using a Peak Detector 5 for AM envelope detectionis Attack Time and Decay Time. Firstly, the signal from IF Amplifier 3 bis AC-coupled into the Peak Detector 5 where the Peak Detector 5 inputis relative to system ground. This removes any dc term from the signalbeing peak-detected, since not doing this will affect the peak detectortime constants.

The Peak Detector 5 employs a fast, closed-loop voltage follower drivingan open-loop envelope detector. Such a construct improves the attacktime of the peak detection function, an important consideration in theSwept LO mode where the IF Filter 4 may only ring-up to its maximumamplitude for 1 or 2 cycles at the IF frequency. Both the voltagefollower and envelope detector circuits are loaded with matched loadaverage load currents to remove any dc error between the voltagefollower and envelope detector.

The current used to set the attack time constant is limited to reducepower supply noise, but otherwise the attack time must be as fast aspossible, since this parameter and IF Filter Q set the upper limit onoperational data rate of the invention. The decay time, however, is afunction of data rate, and the previously mentioned hit rate, whichshould fall between 4 and 10 per bit time, for good system performance.As a result, a switched-capacitor (SC) filter as Baseband Filter 6 waschosen to provide the equivalent resistance-capacitance of the envelopedetector. This allows the use of a much smaller capacitance than wouldotherwise be possible, which allows the Peak Detector 5 to be fullyintegrated.

In addition, the frequency for the SC filter, derived from the ReferenceOscillator Timing Generator 14, allows the time constant to be set withgreat precision. Further, the Timing Generator 14 generates variousharmonically related frequencies that are selectable via decode logic toharmonically modify the Peak Detector 5 decay time constant. This allowsthe receiver to support multiple data rates, simply by selecting whichof a number of frequencies within the Timing Generator is connectedelectronically into the Peak Detector SC filter.

Baseband Filter 6

Once peak detection of the AM envelope is completed by the Peak Detector5, the resulting signal is passed through Baseband Filter 6. TheBaseband Filter 6 is a Switched-Capacitor (SC) filter with a lowpassfrequency characteristic. An SC filter is used to minimize the size ofthe filter capacitors involved, which allows the entire Baseband Filter6 to be integrated onto the integrated circuit. Also, as with the PeakDetector 5 decay time constant, the Filter 6 frequency response can betailored by proper selection of its clock frequency from the TimingGenerator 14. Therefore, since the frequency characteristic of theBaseband Filter 6 is scaled to the output of the Timing Generator 14 andthe output of the Timing Generator is based on the external referencefrequency signal, the characteristics of Filter 6 may be selected bychanging the external reference frequency to meet the user's needs.

SEL0 and SEL1 can be selected to provide four different bandwidthsranging from 600 Hz to 4.8 Khz, depending on the needs of the user.Further, the Peak Detector 5 decay time constant and Baseband Filter 6responses must track as the data rate changes. Thus the clock frequencythat tailors the Baseband Filter 6 response is made identical to thefrequency which tailors the Peak Detector 5 decay time constant,providing a very efficient implementation.

DC Extractor 8

The filtered signal from Baseband Filter 6 contains a dc term related tosignal buffering and an ac term which rides on top of the dc term. Onlythe ac term contains the information of interest. This filtered signalis applied to a very long time-constant lowpass filter, the DC Extractor8. The DC Extractor 8 is made up of an equivalent resistance (orimpedance) constructed on the integrated circuit (IC) using switchedcapacitor techniques. The capacitance C1 for the filter is providedexternally to the IC. The output of the DC Extractor 8 appears at thejunction of the switched resistance and the external capacitor. This isthe sum of the dc term and the average value of the ac term of thesignal that appears at the output of Baseband Filter 6.

This signal is the optimum signal to compare the Baseband Filter outputagainst to decide the presence or absence of a transmitted mark.

The impedance of the DC Extractor 8 is selected to be between1 1600K and200K ohms by the signals SEL0 and SEL1

One may modify the DC Extractor 8 to be a switched-capacitor lowpassfilter. This would allow the capacitor C1 to be integrated onto the IC.This lowers cost and allows the invention to fit into smaller pin-countpackages.

Data Comparator 7

The output signal from the Baseband Filter 6 is compared against theoutput of the DC Extractor 8 by the Data Comparator 7. The resultingoutput, called Data Out, transitions to logic high (or 1) for a marktransmitted to the system (antenna) input within the appropriatefrequency range. Otherwise the output is a logic low (or 0).

The Data Comparator 7 is a CMOS circuit with several important features.The output of the Baseband Filter 6 is a fairly slow moving signal. Toassure that no false switching occurs at the output of the Comparator 7as this signal crosses the comparator threshold, a small amount ofhysteresis or positive feedback is provided across the input stage ofthe Comparator 7. Additionally, the output stage of the Comparator 7 iscurrent limited to minimize switching noise on the power supply.Finally, ac current balancing is employed in the Comparator 7 outputstage to mitigate any ac current term on the power supply.

Reference Oscillator 10 and Timing Generator 14

Reference Oscillator 10

The Reference Oscillator 10 develops the precision timing signal whichis used ubiquitiously throughout the receiver for precise frequency,amplitude, and gain control. The Reference Oscillator 10 design is ofthe Colpitts variety and is connected to a timing device 25 external tothe integrated circuit (IC). The Reference Oscillator 10 design requiresonly a single IC pin for connection to the timing element or signal.Typical timing devices are ceramic resonators, crystals, or (tuned)inductor-capacitor tank circuits. Phase lag capacitors generallyassociated with ceramic resonators are integrated onto the IC to lowercost.

The output from the Oscillator 10 is dc-coupled into a single-endedamplifier which provides differential outputs. The amplifier provides nodc gain. This eliminates any dc offset correction requirements. Theamplifier output is differentially coupled into a comparator whichlimits the output waveform to logic level swings. The output stage ofthe comparator is ac current balanced to eliminate any unnecessary powersupply noise.

Finally, the Reference Oscillator 10 design supports the application ofan external (precision) timing signal rather than a timing element. Thesignal or timing element is applied at the same point on the receiver.

Timing Generator 14

Critical timing signals used by various functions within the inventionare derived within the function Timing Generator 14. The TimingGenerator 14 takes as its input the output of the Reference Oscillator10 and then divides this frequency down to synthesize all otherfrequencies used by the receiver.

A multiplicity of data rates are supported by the receiver with noaddition, deletion, or value modification of external capacitors C1 andC2. This is accomplished by logical (electronic) selection of theappropriate frequencies at the output of the Timing Generator 14function. These selection inputs are preferably fixed in the metal maskof the IC so that the receiver may fit into 8 pins, although thesecontrols can be brought out to the extremities of the invention foradded flexibility.

With the invention in Swept LO mode, the Timing Generator 14 developsthe clock signal (approximately 50 kHz for our particular IF Filtercharacteristics and data rates) which controls the sweep rate of the LOSweep Generator 9 a between fi and fu. For fixed mode, this clock signalis forced statically into one logic state.

The logic circuits used may be conventional in nature.

2Vt Bias Supply 11

The bias supply is provided by the 2Vt Bias Supply 11, which minimizesswitching noise on the power supply and provides isolation of logic andbaseband circuits from sensitive RF and IF circuits.

The 2Vt Bias Supply 11 provides the minimum bias necessary to operateconventional CMOS logic circuits without shoot-through logic switchingcurrents. This allows the fastest operation logic without power supplyswitching noise.

Conceptually, a logic circuit only requires a supply voltage equal tothe sum of a pFET threshold voltage (Vtp) and an nFET threshold voltagewithout impacting logic switching speed. This function is provided by acircuit identified as 2Vt Bias Supply 11 integrated onto the receiver.By passing a small current through a back-to-back diode-connected pFETand nFET, the requisite 2Vt voltage is generated. This voltage isregulated by a CMOS closed-loop voltage follower. This lowers the sourceimpedance of the node which supplies the bias voltage to all logiccircuits. Using such a supply minimizes logic shoot-through currentswithout impacting switching speed. This results in lower logic switchingnoise on the primary supply voltage of the IC and provides isolation ofthe logic switching circuits from the sensitive RF and IF circuits. Thisallows complex logic functions to be integrated on the same IC with lowlevel analog signal processing circuitry to a greater extent than thepresent state of the art.

Alternatively, lower logic supply voltages may be used, although withoutimprovement in switching noise, but only a reduction in logic switchingspeed.

Filter Tuning 13

Variations in semiconductor process parameters and temperatureinvariably result in radical variations in bias currents if some meansof stabilization is not imposed. Filter Tuning 13 (identified asTUNE756K in the transistor level schematics) provides this controlfunction. Filter Tuning 13 is a phase-locked-loop (PLL) based around a756 kHz current-controlled-oscillator (ICO). The reference frequency forthe PLL comes from an appropriate division of the Reference Oscillator10 frequency. Note that the Reference Oscillator 10 frequency isessentially process and temperature independent. The PLL forces theICO's current to whatever value is necessary to yield frequency lockwith the Reference Oscillator 10, irrespective of temperature or processvariations. This yields a bias current whose value is countercompensated for process and temperature variations.

This current is then mirrored and scaled appropriately and used as thebias currents for the LO Sweep Generator 9 a, the IF Filter 4, and theIF Amplifiers 3 a/3 b.

The oscillator and amplifier within this function are fully differentialand balanced. The amplifier is constructed with a bandpasscharacteristic, which exhibits no dc gain. Thus the amplifier andoscillator are dc-coupled without the need for de offset correction orcoupling capacitors. The oscillator is a gm-c type circuit, whichminimizes the capacitance required to construct the oscillator.Oscillator variances are minimized as only the gm of the circuit varieswidely; capacitance is much better behaved over process and temperature.

AGC 12

In order to keep the system operating linearly over a broad range ofinput signal amplitudes, automatic gain control (AGC) 12 is integratedinto the invention. A precision, bandgap reference 15 voltage is appliedto the reference input of a CMOS comparator within the AGC function. Theother input of the comparator monitors the output of the Peak Detector5. When the Peak Detector 5 voltage goes above the reference voltage,the comparator output goes to one logic state, otherwise the output isin the other logic state. The Peak Detector 5 is the optimum location inthe system to monitor the signal level for AGC control from thestandpoint of loop stability.

The comparator output thus engages an electronic switch. This switcheither connects a pushing current source into an externally appliedcapacitor, or connects a pulling current source to the capacitor,depending on the comparator's output state. The capacitor acts as anintegrating function, building a control voltage which is applied to theIF Amplifiers 3 a/3 b and the RF Mixer 2 to control their respectivevoltage gains. The attack time is set to be much faster than the decaytime of the AC control voltage. This is accomplished by making thecurrent of the pushing current source much larger than that of thepulling current source.

An alternative embodiment of the invention is the use of a limiting logdetector rather than translating the RF carrier to an IF frequency whichis filtered. The benefit of this embodiment is that it mitigates therequirement for AGC. There is a penalty however, namely that such asystem has no LO. Hence one cannot build the swept frequency systemdescribed above.

ALTERNATIVE EMBODIMENTS AND ENHANCEMENTS

Use of CMOS technology, balanced differential circuitry, full functionintegration, and the construction of a 2Vt Bias Supply 11 allowenhancements to the above described receiver. Specifically, suchtechniques allow one to further integrate logic functions on thereceiver, which result in lower overall receiver/decoder system costs.Two enhancements worthy of identification are:

(a) integration of the present receiver with a dedicated or fixed(logic) state decoder 30, and

(b) integration of the present receiver with a generic Arithmetic LogicUnit (ALU) or microprocessor type function, or other type ofprogrammable logic function to provide additional decision and/orcontrol functions. Such a function is also represented by the element30.

A decoder is a circuit function that compares an incoming data streamagainst an expected data stream. If the incoming stream is as expected,usually at least one output is activated in a digital system to signifythe matching event. Some decoder systems also transfer a data pattern atthe output upon matching the expected data stream. In high securityapplications, the encoding and decoding operation may involve a rollingor changing code scheme. These schemes virtually eliminate one's abilityto copy a transmitted data stream and then use that same data streamlater to activate the decoder.

With rolling code schemes, the addition of an electrically programmableand erasable memory is generally required to update the code.

The combined integration of the receiver with any or all of the decoderschemes with and without variable or fixed memory become economicallyfeasible since the technology is the same. As semiconductor technologycontinues to advance, these combinations continue to increase in value.

Other embodiments include the addition of the following functions:

programmable squelch offset on the Data Comparator 7 slicing threshold,and

shutdown mode for low-power duty-cycling of the receiver; and

using the AGC level to provide an indication of the received signallevel. This signal may be monitored for ranging and distance checking.

By implementing the receiver in CMOS, it is economical to includedecoder functions on the same IC since most of the applications for thistype of radio include a decoder after the radio receiver. The decode canbe either fixed or variable (i.e., programmable, like a microprocessor).This receiver is primarily targeted towards the following applications:

Automotive Keyless Entry

Garage Door Openers

Home Keyless Entry

Security Systems

Remote Door Bell Ringers

Obviously, their are numerous other applications that combine a simpleAM type receiving radio with a digital decoding system.

This receiver's uniqueness is further exemplified by the observationthat a complete RF radio system can be built with no other external RFcomponents except an antenna. By complete integration of the entire RFsignal path, the RF emissions of the device and the composite system arereduced to levels which greatly simplify, if not completely eliminate,international regulatory compliance requirements. These requirementsvary from country to country, but are similar to the United States FCCPart 15 regulations for periodic band operation. The emissions are loweras a result of the small radiating elements associated with a fullyintegrated system. Current art uses, for example, an external inductorto build the local oscillator. This external component has a largerradiating element which must generally be shielded to solve theemissions problem.

The techniques in this receiver can be extended to many frequency bandsof operation by scaling the device and interconnect layout geometries toaccommodate advances in semiconductor processing. For example, in theUnited States, the FCC has allocated a band of frequencies near 900 MHzfor unlicensed radios. This band is commonly referred to as an ISM band.The receiver applied to this and other bands would allow for fullyintegrated data modems, not just encoder/decoder systems. Continuoussignals can also be received by the system described in this receiver.An on-chip A/D converter can convert the analog received signal to adigital counterpart for processing with an on-chip digital computingelement similar to a microprocessor.

Any circuitry not expressly described herein may be conventional, andcircuitry need not be described for a complete understanding of theinvention by those skilled in the art.

Schematic Diagrams

Schematic Diagrams of the various functional units in FIG. 1 areprovided as FIGS. 4-76. These schematic diagrams are easily understoodby those skilled in the art. All circuitry is formed on a single chip.

CONCLUSION

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of this invention.

1. A method for operating a radio receiver formed as a monolithicintegrated circuit (IC), the method comprising: generating a referencefrequency using a reference oscillator in the monolithic IC, thereference frequency being process and temperature independent; andgenerating a bias signal from a bias signal generator in the monolithicIC using the reference frequency, the bias signal controlling thecharacteristics of various components within the radio receiver, and thebias signal having a value that is compensated for temperature andprocess variations.
 2. The method of claim 1, further comprising:generating a local oscillator signal using a local oscillator in themonolithic IC; mixing an input radio frequency (RF) signal with thelocal oscillator signal using a mixer in the monolithic IC to generate afrequency translated signal; amplifying the frequency translated signalusing an intermediate frequency (IF) amplifier in the monolithic IC;filtering the frequency translated signal using an IF filter in themonolithic IC to generate an IF filtered signal; and biasing the localoscillator, the mixer, the IF amplifier, and the IF filter using thebias signal to achieve temperature and process independence for thelocal oscillator, the mixer, the IF amplifier, and the IF filter,respectively.
 3. The method of claim 2, further comprising: monitoringthe IF filtered signal using an automatic gain control (AGC) circuit inthe monolithic IC to generate a first AGC signal when a magnitude of theIF filtered signal is greater than a reference voltage; and reducing again of the IF amplifier in response to the first AGC signal.
 4. Themethod of claim 1, wherein generating the bias signal comprises:adjusting a current provided by a current-controlled-oscillator (ICO) ina phase-locked-loop (PLL) in the bias signal generator until frequencylock is achieved between the PLL and the reference frequency; andsupplying the current provided by the ICO as the bias signal.